The present invention relates to a display device driver IC (i.e., an integrated circuit for driving a display device) for applying drive signals to electrodes of a display device, and particularly a liquid crystal device driver IC having drive signal output terminals having improved drive performances.
Hitherto, for driving a liquid crystal device having electrodes arranged in a matrix form, a driver IC for supplying drive signals to the electrodes is designed to have a plurality of terminals having equal drive capacities.
Incidentally, the drive of a liquid crystal panel comprising matrix electrodes as an example of conventional liquid crystal device along a signal electrode (a scanning electrode or a data electrode) constituting the matrix electrodes is electrically equivalently represented by a ladder circuit as shown in FIG. 16. Now, if the resistance and capacitance per unit length of the matrix electrode or signal electrode are denoted by r and c, respectively, and the overall resistance and capacitance along the matrix electrode are denoted by R and C, respectively, a voltage waveform V appearing at a point B in response to a voltage input V0 applied to a point A of the ladder circuit is given as a solution of the following partial differential formula:                     ∂        2            ⁢      V              ∂              x        2              =      rc    ⁢                            ∂                      V            0                                    ∂          t                    .      
The solution is expressed as follows.                               V                      V            0                          =                  xe2x80x83                ⁢                              -                          xe2x80x83                        ⁢                          4              π                                ⁢                                    ∑                              n                =                0                            ∞                        ⁢                                                                                (                                          -                      1                                        )                                    n                                                                      2                    ⁢                                          xe2x80x83                                        ⁢                    π                                    +                  1                                            ⁢              exp              ⁢                              xe2x80x83                            ⁢                              (                                                      -                                                                  (                                                                              (                                                                                          2                                ⁢                                n                                                            +                              1                                                        )                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      π                            /                            2                                                                          )                                            2                                                        ⁢                                      t                    /                    CR                                                  )                                                                            =                  xe2x80x83                ⁢                  1          -                      xe2x80x83                    ⁢                                    4              π                        ⁢                          (                              (                                                      exp                    ⁢                                          xe2x80x83                                        ⁢                                          (                                              -                                                  xe2x80x83                                                ⁢                                                                                                            π                              2                                                        ⁢                            t                                                                                4                            ⁢                            CR                                                                                              )                                                        -                                                            1                      3                                        ⁢                    exp                    ⁢                                          xe2x80x83                                        ⁢                                          (                                              xe2x80x83                                            ⁢                                                                        3                          ⁢                                                      π                            2                                                    ⁢                          t                                                                          4                          ⁢                          CR                                                                    )                                                        +                  …                                                                        
The above formula provides plots of relative voltage V/V0 versus time (on a scale of time constant CR) as shown in FIG. 17.
Now, in a region of t greater than CR, the second term and so on can be negligible as sufficiently small, so that a time t0 in which voltage response reaches 90% of the input (V/V0=0.9) can be approximately represented by the following equation:       V          V      0        =      0.9    =          1      -              xe2x80x83            ⁢                        4          π                ⁢                  xe2x80x83                ⁢        exp        ⁢                  xe2x80x83                ⁢                  (                      -                          xe2x80x83                        ⁢                                                            π                  2                                ⁢                                  t                  0                                                            4                ⁢                CR                                              )                    
The above equation can be converted as follows:
0.1=(4/xcfx80)xc2x7exp(xe2x88x92xcfx802t0/4CR)
xcfx80/40=exp(xe2x88x92xcfx802t0/4CR).
By taking natural logarithm of both sides,
ln(xcfx80/40)=xe2x88x92xcfx802t0/4CR
t0=xe2x88x92(4/xcfx802)ln(xcfx80/40)xc2x7CR.
As
xe2x88x92(4/xcfx802)=ca. xe2x88x9241,
and
xe2x80x83ln(xcfx80/40)=ca. xe2x88x922.5,
the above equation is reduced to
t0=ca. CR.
Thus, a time t0 in which a voltage output at the remotest point rises up to 90% of the input voltage, i.e., a 0-90% time constant can be expressed by a product of the wiring resistance (R) and the capacitance (C).
The above calculation is based on an assumption that the drive capacity of a driver IC is infinitely large, but the drive capacity of an actual driver IC is limited, so that the time constant, i.e., a rise time, depends on the capacity.
A driver IC has an on-resistance which varies depending on operation points so that the drive capacity exhibits a non-linear characteristic. However, in order to obtain a time constant of drive waveform, the drive capacity is generally approximated as a linear characteristic based on a constant on-resistance Ron.
Accordingly, a 0-90% time constant t0-90 when a panel represented by the equivalent circuit shown in FIG. 16 is driven by a diver IC having an on-resistance Ron is calculated as follows.
t0-90=C(R+Ron).
Incidentally, a driver IC is designed to have an on-resistance Ron so that the 0-90% time constant t0-90 satisfies a required standard.
Conventionally, driver ICs 40 for driving a panel having matrix electrodes including data signal electrodes S and scanning signal electrodes C as shown in FIG. 18 have been designed to have equal on-resistances Ron at the respective drive signal output terminals. This is because loads determined by a combination of capacitances along data signal electrodes S or scanning signal electrodes C with wiring resistances are equal for the respective data signal electrodes and for the respective scanning signal electrodes.
Further, as the capacitances and wiring resistances of the data signal electrodes S and the scanning signal electrodes C, respectively, vary depending on pixel arrangements and sizes of respective panels, the driver ICs 40 have been designed and produced for each pixel having a different pixel arrangement.
On the other hand, in the case of a liquid crystal device including electrodes of different widths for realizing areal gradational display as shown in FIG. 19, electrodes S1 and S2 having different widths have mutually different capacitances and wiring resistances (and also electrodes C1 and C2 do).
Now, drive voltage responses are considered when such electrodes having different widths are supplied with drive signals from driver ICs 40 having equal capacities. For example, when a scanning electrode C1 of a narrower width having a capacitance CS and a resistance RS is driven by a driver IC 40 having an on-resistance Ron as shown in FIG. 20, the response at the remotest point from the IC 40 results in a waveform as shown in FIG. 21. On the other hand, when a scanning electrode of a broader width having a capacitance 4CS and a resistance RS/4 is driven by a driver IC 40 having also an on-resistance Ron as shown in FIG. 22, the response at the remotest point from the IC 40 results in a waveform as shown in FIG. 23.
The 0-90% time constant Ta0-90 and Tb0-90 in the drive waveforms shown in FIGS. 21 and 23, respectively, approximately calculated as follows:
Ta0-90=CSxc3x97(Ron+RS)=CSxc2x7Ron+CSxc2x7RS
Tb0-90=4CSxc3x97(Ron+RS/4)=4CSxc2x7Ron+CSxc2x7RS
∴Tbxe2x88x92Ta=3CSxc2x7Ron
Thus, the drive of a broader electrode C2 requires a response time (rise time or fall time) which is longer by 3CSxc2x7Ron than the drive of a narrower electrode C1.
As a result, the energies applied to the liquid crystal via a broader electrode and a narrower electrode can be different from each other, resulting in a substantial difference in picture display quality.
On the other hand, as picture display quality can also be degraded in the case where less energy is applied to a liquid crystal, the on-resistance of driver ICs for driving electrodes of different widths is set to be suitable for driving electrodes of broader widths. When using driver ICs having an on-resistance Ron suitable for a broader electrode, however, these driver ICs are liable to cause difficulties driving a narrower electrode, such as a larger current flow through the narrower electrodes resulting in fluctuation of power supply potential or ground potential for the liquid crystal device, occurrence of radiation noise, heat generation, and increase in current consumption.
Further, in designing and production of driver ICs, an additional area is required for output transistors and is liable to occupy the largest area on a chip, so that a larger semiconductor chip is required to incur a cost increase.
In order to obviate difficulties, such as a lowering in picture display quality, fluctuation of power supply potential or ground potential, occurrence of radiation noise, heat generation and an electric current consumption, the drive capacities of driver ICs have to be optimized, so that development of driver ICs has been effected for each panel size.
As a result, designing and development of a diversity of driver ICs have been required so as to comply with a diversity of display panels requiring special driver ICs exclusively designed and developed therefor, thus having incurred increases in period and cost for development.
In view of the above-mentioned problems of the prior art, a principal object of the present invention is to provide a display device driver IC allowing simple designing and development and yet capable of preventing an occurrence of fluctuation in display quality.
According to the present invention, there is provided a driver IC (integrated circuit) for supplying drive signals to a plurality of signal electrodes of a display device for driving the display device, wherein said driver IC comprises a plurality of drive signal output terminals having drive capacities which vary depending on loads of respective signal electrodes of the display device to which the output terminals are connected so as to supply the respective signal electrodes of the display device with drive signal waveforms having identical time constant.
According to another aspect of the present invention, there is provided a driver IC for supplying drive signals to a plurality of signal electrodes of a display device for driving the display device, wherein said driver IC comprises a plurality of drive signal output terminals arrange to have variable drive capacities which vary depending on loads of respective signal electrodes of the display device to which the output terminals are connected so as to supply the respective signal electrodes of the display device with drive signal waveforms having identical time constant.
Preferably, the driver IC are designed to include a number of juxtaposed transistors corresponding to but larger in number than the drive signal output terminals, and the respective drive signal output terminals are connected to prescribed numbers of transistors so as to have different drive capacities depending on loads of the signal electrodes of the display device to which the output terminals are connected.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.